The invention relates to a method for checking whether documents have been separated from opened envelopes, in which each envelope is conveyed along at least one input transducer, which measures a characteristic of that envelope along a measuring path extending over that envelope.
Such a method is disclosed in U.S. Pat. No. 4,016,708. In this known method an opened envelope which has been subjected to an operation for separating therefrom the documents received therein, is passed along two detectors arranged at a distance from each other in the direction of conveyance, each detector comprising a light source and a photocell arranged on the other side of the transport path, opposite the associated light source. The photocells are each adapted for activating an associated logic circuit if the amount of light received falls below an amount of light that passes through the greater part of a completely emptied envelope. False alarms as a consequence of reduced transparency at the location of adhesive edges, printings, and the like are avoided in that an envelope-suspect signal is produced only when both detectors activate the logic circuit simultaneously.
A drawback of this method is that it is only suitable for processing substantially identical return mail. In the processing of different envelopes that allow different amounts of light to pass, on the one hand the presence of a document is signaled erroneously when thick and dark envelopes pass the detectors and on the other hand the presence of a document is not signaled when relatively transparent envelopes with a relatively transparent document pass the detectors.
U.S. Pat. No. 4,113,105 discloses a method for verifying the empty status of envelopes, in which the signals coming from three light-sensitive sensors arranged along a line directed transversely to the direction of conveyance are compared with each other and an envelope-suspect signal is produced if the signal of the central sensor differs from that of the lower or upper sensor for a specified minimum period of time. Moreover, the signal of the central sensor is compared with a specified value and an envelope-suspect signal is produced if this comparison reveals undesired differences.
In this method envelopes and documents of different transparencies can be processed in a random order, but documents of such width that they pass under the central as well as the upper and lower sensors are detected only if the total transparency decreases to such an extent that the comparison of the signal produced by the central sensor with the above-mentioned specified value is a reason for that. This disadvantage is of major importance because documents are generally mailed in envelopes into which they fit with little clearance. A further drawback of this method is that it is only suitable for processing envelopes having one specified dimension transverse to the direction of conveyance, because the upper and the lower sensor must pass closely along the edge of the envelope. A yet further drawback of this method is that when a thick and/or dark envelope passes the sensors, an envelope-suspect signal is also produced if no document is being carried along with such envelope but the comparison of the signal produced by the central sensor with the specified value mentioned does provide a reason for it.
International patent application WO 88/01543 discloses a method for the verification of the empty status of envelopes, in which the thickness of the envelopes is measured. This measured thickness is compared with a value determined during a test cycle. In this method, too, it is not properly possible to process different envelopes in random order because they may have different thicknesses. The envelopes to be processed must have the same thickness as the envelopes supplied in the test cycle.